The present invention relates to a visible buried semiconductor laser device manufactured by the mixed-crystal forming technique based on impurity diffusion.
When an impurity such as Zn is thermally diffused or ion planted into the superlattice, which consists of extremely thin semiconductor layers, several to several tens nm thick, of different compositions such as AlGaAs--GaAs, the superlattice is destroyed and transformed into a uniform mixed-crystal. This fact is known. When the mixed-crystal is formed, refractivity, forbidden band, and the like are varied. This nature can be used for confining light and carriers therein. The buried semiconductor laser device, manufactured by the mixed-crystal forming technique based on impurity diffusion, has various advantages: low threshold current value, high efficiency, stable transverse mode, and easy integration. For this reason, AlGaAs series as a material for the near-infrared semiconductor laser is frequently used for the buried semiconductor laser device.
Appl. Phys. Lett. 54, p2136 (1989) describes the mixed-crystal forming technique based on the impurity diffusion in A1GaInP series as a material for the visible semiconductor laser device, in which the impurity is Zn. When Zn is diffused, a natural superlattice of A1GaInP as a material for the visible semiconductor laser device is transformed into a mixed-crystal, but mutual diffusion of Al and Ga is not remarkable. The natural superlattice follows. When A1GaInP is crystal grown by a MOCVD method, the group III atoms are regularly arrayed into the natural superlattice. This phenomenon is described in Appl. Phys. No. 9 of Volume 58, p1360 (1989). Where the natural superlattice is transformed into a mixed-crystal, the forbidden band width difference for confining carriers and the refractivity difference for confining light cannot be rendered satisfactorily large between a mixed-crystal region and a non-mixed-crystal region.
To efficiently confine carriers, it is necessary to greatly vary the forbidden band width and the refractivity between a mixed-crystal region and a non-mixed-crystal region, by causing Al and Ga to mutually diffuse.
Journal of Applied Physics, vol. 66, p482 (1989) describes a semiconductor laser device manufactured by the mixed-crystal forming technique based on the Si diffusion in AlGaInP series. A Si-diffusion basis, buried visible semiconductor laser device is simpler manufacture than the ridge-stripe type visible semiconductor laser device.
The laser described in the paper has poor laser characteristics since during the manufacturing process, particularly during the Si diffusing process, many dislocations and defects are caused. Our observation by a transmission electron microscopy showed that these dislocations and defects are concentrated mainly in the surface region of the second conductivity type GaInP intermediate layer in the Si diffused region.
The influence of the dislocations and defects on the semiconductor laser device will be described. FIG. 9 shows an AlGaInP buried laser, which is manufactured by the mixed-crystal forming technique based on the Si impurity diffusion disclosed in Published Unexamined Japanese Patent Application No. Hei. 6-53604. In the figure, reference numeral 401 designates an n-type side electrode; 402, an n-type GaAs substrate; 403, an n-type GaInP buffer layer; 404, an n-type AlInP cladding layer; 405, a GaInP active layer; 406, a p-type AlInP cladding layer; 407, a p-type GaInP intermediate layer; 408, an Si diffusion source film; 409, an SiO.sub.2 diffusion protective film/current block layer; 410, a p-type side electrode; 411, a p.sup.+ type GaAs contact layer; 412, an Si diffusion region; and 413, a surface region where many dislocations and defects that are caused by the Si diffusion are present.
In this type of the AlGaInP buried laser by the Si diffusion, ideally, the current injected from the p-type side electrode 410 is squeezed by the pn junction formed along the boundary between the p-type GaInP intermediate layer 407 and the Si diffusion region 412. As a result, the current is efficiently injected into only the active layer.
Actually, the current leaks through the boundaries 414 and 415 each between the surface region 413 where many dislocations and defects are present and the p-type GaInP intermediate layer 407. In this situation, the current is inefficiently injected into the active layer. The result is high threshold current value, low efficiency,. and poor temperature characteristic.